efi/libstub/x86: Use const attribute for efi_is_64bit()

Reshuffle the x86 stub code a bit so that we can tag the efi_is_64bit()
function with the 'const' attribute, which permits the compiler to
optimize away any redundant calls. Since we have two different entry
points for 32 and 64 bit firmware in the startup code, this also
simplifies the C code since we'll enter it with the efi_is64 variable
already set.
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200113172245.27925-2-ardb@kernel.org

arch/x86/boot/compressed/eboot.c

@@ -21,16 +21,18 @@
 #include "eboot.h"
 
 static efi_system_table_t *sys_table;
-static bool efi_is64 = IS_ENABLED(CONFIG_X86_64);
+extern const bool efi_is64;
 
 __pure efi_system_table_t *efi_system_table(void)
 {
 	return sys_table;
 }
 
-__pure bool efi_is_64bit(void)
+__attribute_const__ bool efi_is_64bit(void)
 {
-	return efi_is64;
+	if (IS_ENABLED(CONFIG_EFI_MIXED))
+		return efi_is64;
+	return IS_ENABLED(CONFIG_X64_64);
 }
 
 static efi_status_t
@@ -710,8 +712,7 @@ static efi_status_t exit_boot(struct boot_params *boot_params, void *handle)
  */
 struct boot_params *efi_main(efi_handle_t handle,
 			     efi_system_table_t *sys_table_arg,
-			     struct boot_params *boot_params,
-			     bool is64)
+			     struct boot_params *boot_params)
 {
 	struct desc_ptr *gdt = NULL;
 	struct setup_header *hdr = &boot_params->hdr;
@@ -721,9 +722,6 @@ struct boot_params *efi_main(efi_handle_t handle,
 
 	sys_table = sys_table_arg;
 
-	if (IS_ENABLED(CONFIG_EFI_MIXED))
-		efi_is64 = is64;
-
 	/* Check if we were booted by the EFI firmware */
 	if (sys_table->hdr.signature != EFI_SYSTEM_TABLE_SIGNATURE)
 		goto fail;

arch/x86/boot/compressed/head_64.S

@@ -211,10 +211,9 @@ SYM_FUNC_START(startup_32)
 	movl	efi32_boot_args(%ebp), %edi
 	cmp	$0, %edi
 	jz	1f
-	leal	handover_entry(%ebp), %eax
+	leal	efi64_stub_entry(%ebp), %eax
 	movl	%esi, %edx
 	movl	efi32_boot_args+4(%ebp), %esi
-	movl	$0x0, %ecx
 1:
 #endif
 	pushl	%eax
@@ -242,6 +241,7 @@ SYM_FUNC_START(efi32_stub_entry)
 	movl	%ecx, efi32_boot_args(%ebp)
 	movl	%edx, efi32_boot_args+4(%ebp)
 	sgdtl	efi32_boot_gdt(%ebp)
+	movb	$0, efi_is64(%ebp)
 
 	/* Disable paging */
 	movl	%cr0, %eax
@@ -452,8 +452,6 @@ SYM_CODE_END(startup_64)
 	.org 0x390
 SYM_FUNC_START(efi64_stub_entry)
 SYM_FUNC_START_ALIAS(efi_stub_entry)
-	movq	$1, %rcx
-SYM_INNER_LABEL(handover_entry, SYM_L_LOCAL)
 	and	$~0xf, %rsp			/* realign the stack */
 	call	efi_main
 	movq	%rax,%rsi
@@ -632,6 +630,7 @@ SYM_DATA_END_LABEL(gdt, SYM_L_LOCAL, gdt_end)
 
 #ifdef CONFIG_EFI_MIXED
 SYM_DATA_LOCAL(efi32_boot_args, .long 0, 0)
+SYM_DATA(efi_is64, .byte 1)
 #endif
 
 /*

arch/x86/include/asm/efi.h

@@ -221,7 +221,7 @@ efi_status_t efi_set_virtual_address_map(unsigned long memory_map_size,
 
 /* arch specific definitions used by the stub code */
 
-__pure bool efi_is_64bit(void);
+__attribute_const__ bool efi_is_64bit(void);
 
 static inline bool efi_is_native(void)
 {